What about renaming current GDP into GDP Growth, and making new property, called GDP - but instead of simulation value like GDP now it would be numbers, something like debt or revenues, that can be changed from -10% to +10% per turn (or +-20% per year at max).

For example, if your current policies set your GDP Growth exactly at 50%, then GDP won't change at all.Or if you manage to get GDP Growth at 100%, then value of GDP would increase 10% per turn, possibly overheating your country.